Testing of Contact Surface Layers

From Electrical Contacts
Revision as of 15:33, 8 January 2014 by Ermisch (talk | contribs) (Created page with "===13.2.1=== Surface Layer Properties ====13.2.1.1 Layer Thickness==== The primary requirement on an electroplated deposit is its thickness since a series of other propertie...")

(diff) ← Older revision | Approved revision (diff) | Latest revision (diff) | Newer revision → (diff)
Jump to: navigation, search

===13.2.1=== Surface Layer Properties

13.2.1.1 Layer Thickness

The primary requirement on an electroplated deposit is its thickness since a series of other properties such as porosity, hardness, and ductility, are depending on the layer thickness. A widely used method for thickness measurement is the one using microscopical evaluation of a mounted micro section. Because of optical resolution limits however, layer thicknesses below 0.5 μm cannot be measured with sufficient accuracy. For low coating thicknesses the X-ray fluorescence method is used which often is integrated for quality assurance into the manufacturing process, for example in selective reelto- reel electroplating plating lines. In this process the returned characteristic xray impulses of the coating material and the substrate, generated by the primary x-rays, are detected and counted and then converted into material thickness by a computerized indicator or recording and control unit.

13.2.1.2 Porosity

Pores are surface defects which may have multiple causes. These include roughness and defects in the substrate layer or material such as grooves or scratch marks, as well as cracks in the base material which may have been generated by bend stresses or mechanical wear (Fig. 13.1).

At the foot points of the pores the substrate material is exposed to the surrounding atmosphere. This can cause corrosion products to rise through the pores to the contact surface, expand there further, and thus lead to increased contact resistance. The allowable number of pores in gold layers, for example for connectors, mainly depends on the concentration of corrosive gases in the intended working environment.

Most methods of porosity testing are based on detecting the substrate material which is transferred to the surface during an electrochemical treatment process. For testing of gold and palladium coatings on nickel containing substrates the dimethyl-glyoxin test for detecting the nickel has been a proven method. The electrochemical methods are mostly working on a electrographic or electrolytic basis.

In the electrographic test a strip of filter material saturated with an electrolyte or a gelatin foil is pressed onto the sample. After the application of an electrical field and current the reaction products are made visible by chemical indicators.

During the electrolytic test a sample is immersed into an electrolyte containing a indicator solution. After passing electrical current the pores are visible as colored spots.

Frequently a SO test at higher concentrations (100 ppm) and high humidity 2 levels (95% RH at 40°C) is used. One advantage of this method is that the severity level can be increased easily by varying the concentration of SO2. Besides these, other corrosive gas mixtures of H2S, SO2, and NO2 are used in porosity tests (i.e. tests according to ASTM B735 and B 799).

Fig. 13.1: Porosity of an electroplated hard gold layer as a function of the layer thickness at different surface roughness values Ra

13.2.1.3 Hardness

The hardness of electroplated surface coatings depends on their deposition parameters and therefore on the structure and concentration of incorporated substances. The hardness measurement is however not a true indicator of the mechanical properties such as frictional wear characteristics like for clad meltmetallurgically produced layers. This is caused by their fundamentally different structure compared to alloys. Brittleness and internal stresses also have an influence on the hardness.

Usually hardness is measured in Europe and Asia applying the Vickers scale (DIN EN ISO 6507-1) for which an indenter of a symmetrical diamond shape is used. For thin layers the micro-hardness is measured in a metallographic crosssectional mount (DIN ISO 416).

In the US the hardness values are often reported by values of the Knoop scale. This method uses a asymmetrical diamond indenter on the layer producing a rhombic indentation picture. Values obtained by both methods are comparable but cannot be converted into each other easily.

During hardness measurements the indentation depth should not exceed 10% of the layer thickness. Depth of up to 30% can be accepted only if the hardness values of the layer and substrate are in a similar range. For layers of pure metals the hardness of the base material can influence the one for the coating in the immediate boundary area. A decrease in hardness can then be observed towards the layer's surface.

13.2.1.4 Ductility

The ductility indicates how much a coating layer can be plastically deformed without cracking. Therefore it is an important measure for the quality of electroplated coatings. If the ductility is too low, cracks can develop in the layer. This crack formation can occur due to internal stresses right after the deposition or develop from mechanical stressing during subsequent mechanical deformation.

To evaluate the ductility of gold layers, a bending test according to DIN 50 153 is usually employed. For certain applications the testing method is agreed upon between the coating manufacturer and the user. After bending the test sample over a pre-defined radius the surface layer in the bend area is examined microscopically. The detection of cracks or even delaminating is an indication of insufficient ductility